The pollution level of a River is measured:

CONCEPT: River (water) pollution from organic waste is quantified by measuring how much dissolved oxygen is consumed by microorganisms while decomposing organic matter present in a water sample, over a fixed incubation period (conventionally 5 days at 20 degrees Celsius). This oxygen-consumption test is called the Biochemical Oxygen Demand (BOD) test; a higher BOD value means microorganisms are consuming more oxygen to break down organic waste, which signals a heavier organic pollution load and leaves less dissolved oxygen for aquatic life.

APPLICATION: The stem asks which of the four given terms denotes the standard scientific parameter used to measure a river's pollution level. Regulatory and monitoring bodies such as India's Central Pollution Control Board (CPCB) classify river-stretch pollution using BOD readings, since BOD directly reflects the organic contamination present in the water.

CROSS-CHECK (each remaining term traced to its own separate meaning, unrelated to pollution measurement):

• ATP (Adenosine Triphosphate) is the energy-currency molecule used inside living cells for metabolic reactions — a cell-biology term, not an environmental-monitoring parameter.

• STP commonly denotes Standard Temperature and Pressure (a fixed reference condition for gas measurements) or, in a different context, a Sewage Treatment Plant (a facility) — neither is itself a measurable pollution index.

• WPL is not a recognized scientific or regulatory pollution-measurement parameter; no standards body defines a test by this name.

RESULT: Since only the oxygen-consumption test described above is an established, quantifiable water-pollution parameter, the river's pollution level is measured by BOD.